This invention relates generally to a system for controlling motor speed and more particularly, to a system for accurately controlling high speed optical shafts.
Although this invention may be used for speed control of any rotating shaft, it has been shown to have particular application in high speed cameras.
High speed cameras operate on a principal of rapidly passing the film past an aperture for exposure rather than utilizing a conventional shutter mechanism. Film thus moving requires precisely controlled motors for driving the various parts including take up rolls. These motors have shafts which turn on the order of 250 revolutions per second and have absolute minimum deviation from this speed.
Typically, an optical shaft encoder is connected to the shaft for measuring deviation in speed. The optical shaft encoder consists of a photographic fixed mask and a corresponding rotating disk attached to the shaft. The mask consists of alternating transparent and opaque areas. Light is directed through the photographic mask and the rotating disk which forms a Miore interference pattern which is then detected into an appropriate photo sensor, generating an electrical sine wave as output.
In the past, in an effort to increase the accuracy of such a system, the improvement has been directed toward the photographic mask. Increasing the number of transparent areas increases the sampling and hence the quantity of sine waves as output. However, there are two major drawbacks to this approach; first, such masks are very expensive to manufacture. Second, they are limited by the nature of the art as to how many alternating areas they can accommodate.
Hence, a new and improved system for measuring and controlling shaft speed is needed which does not include attempting to redesign or further modify existing mask structures. Utilizing a conventional relatively inexpensive mask and optical shaft encoder, the invention presented hereinafter utilizes an improved electronic system to enhance the capabilities of the encoder for motor shaft speed control.